Pain is an unpleasant sensation varying in severity in a local part of the body or several parts of the body resulting from injury, disease, or emotional disorder. Pain can be classified according to its duration. Acute pain, which lasts less than one month, usually has a readily identifiable cause and signals tissue damage. In addition, acute pain syndromes can be episodic, for example recurrent discomfort from arthritis. Chronic pain can be defined as pain that persists more than one month beyond the usual course of an acute illness or injury, or pain that recurs at intervals over months or years, or pain that is associated with a chronic pathologic process. In contrast to acute pain, chronic pain loses its adaptive biologic function. Depression is common, and abnormal illness behavior often compounds the patient's impairment.
Millions of people suffer from chronic or intractable pain. Persistent pain varies in etiology and presentation. In some cases, symptoms and signs may be evident within a few weeks to a few months after the occurrence of an injury or the onset of disease, e.g. cancer or AIDS. Like many illnesses that at one time were not well understood, pain and its many manifestations may be poorly treated and seriously underestimated. Inappropriately treated pain seriously compromises the patient's quality of life, causing emotional suffering and increasing the risk of lost livelihood and disrupted social integration. Severe chronic pain affects both the pediatric and adult population, and often leads to mood disorders, including depression and, in rare cases, suicide.
In the last several years, health policy-makers, health professionals, regulators, and the public have become increasingly interested in the provision of better pain therapies. This interest is evidenced, in part, by the U.S. Department of Health and Human Services' dissemination of Clinical Practice Guidelines for the management of acute pain and cancer pain. There is currently no nationally accepted consensus for the treatment of chronic pain not due to cancer, yet the economic and social costs of chronic pain are substantial, with estimates ranging in the tens of billions of dollars annually.
Three general classes of drugs are currently available for pain management, nonsteriodal anti-inflammatories, opioids, and adjuvant analgesics. The nonsteriodal anti-inflammatories class includes drugs such as aspirin, ibuprofen, diclofenac, acetaminophen, celecoxib, and rofecoxib. The opioid class includes morphine, oxycodone, fentanyl, and pentazocine. Adjuvant analgesics include various antidepressants, anticonvulsants, neuroleptics, and corticosteroids.
Opioids are the major class of analgesics used in the management of moderate to severe pain because of their effectiveness, ease of titration, and favorable risk-to-benefit ratio. Opioids produce analgesia by binding to specific receptors both within and outside the CNS. Opioid analgesics are classified as full agonists, partial agonists, or mixed agonist-antagonists, depending on the receptors to which they bind and their intrinsic activities at each receptor.
Three subclasses of opioid receptor have been identified in humans, namely the δ-, κ-, and μ-opioid receptors. Analgesia is thought to involve activation of μ and/or κ receptors. Notwithstanding their low selectivity for μ over κ receptors, it is likely that morphine and morphine-like opioid agonists produce analgesia primarily through interaction with μ receptors; selective agonists of κ receptors in humans produce analgesia, because rather than the euphoria associated with morphine and congeners, these compounds often produce dysphoria and psychotomimetic effects. The consequences of activating δ receptors in humans remain unclear.
Although opioids can be very effective in pain management, they do cause several side effects, such as respiratory depression, constipation, physical dependence, tolerance, and withdrawal. These unwanted effects can severely limit their use.
Opioids are known to produce respiratory depression that is proportional to their analgesia. This respiratory depression can be life threatening. This results in a narrow range between the effective dose and a dose that produces respiratory depression. Because of this narrow therapeutic index, patients receiving opioid therapy must be closely monitored for signs of respiratory failure.
Opioids can also cause constipation in individuals receiving them. This side effect can be severe and may require prolonged hospitalization, or even surgical intervention.
Commonly used full agonists include morphine, hydromorphone, meperidine, methadone, levorphanol, and fentanyl. These opioids are classified as full agonists because there is not a ceiling to their analgesic efficacy, nor will they reverse or antagonize the effects of other opioids within this class when given simultaneously. Side effects include respiratory depression, constipation, nausea, urinary retention, confusion, and sedation. Morphine is the most commonly used opioid for moderate to severe pain because of its availability in a wide variety of dosage forms, its well-characterized pharmacokinetics and pharmacodynamics, and its relatively low cost. Meperidine may be useful for brief courses (e.g., a few days) to treat acute pain and to manage rigors (shivering) induced by medication, but it generally should be avoided in patients with cancer because of its short duration of action (2.5 to 3.5 hours) and its toxic metabolite, normeperidine. This metabolite accumulates, particularly when renal function is impaired, and causes CNS stimulation, which may lead to dysphoria, agitation, and seizures; meperidine, therefore, should not be used if continued opioid use is anticipated.
The development of physical dependence with repeated use is a characteristic feature of the opioid drugs, and the possibility of developing drug dependence is one of the major limitations of their clinical use. Almost all opioid users rapidly develop drug dependency which can lead to apathy, weight loss, loss of sex drive, anxiety, insomnia, and drug cravings. Although physical dependence is common, addiction and abuse are not common in pain patients who are treated appropriately with opioid drugs.
Historically, the development of analgesic tolerance was believed to limit the ability to use opioids efficaciously on a long-term basis for pain management. Tolerance, or decreasing pain relief with the same dosage over time, has not proven to be a prevalent limitation to long-term opioid use. Experience with treating cancer pain has shown that what initially appears to be tolerance is usually progression of the disease. Furthermore, for most opioids, there does not appear to be an arbitrary upper dosage limit, as was once thought.
Cessation of opioid administration may result in a withdrawal syndrome. Symptoms of withdrawal are often the opposite of the effects achieved by the drug; withdrawal from morphine, however, results in complex symptoms that may seem unrelated to its effects. Misunderstanding of addiction and mislabeling of patients as addicts result in unnecessary withholding of opioid medications. Addiction is a compulsive disorder in which an individual becomes preoccupied with obtaining and using a substance, the continued use of which results in a decreased quality of life. Studies indicate that the de novo development of addiction is low when opioids are used for the relief of pain. Furthermore, even opioid addicts can benefit from the carefully supervised, judicious use of opioids for the treatment of pain due to cancer, surgery, or recurrent painful illnesses such as sickle cell disease.
The known opioids have been very effective in pain management. However, they have restricted use because of several potentially severe side effects. Therefore, there is a current need for pharmaceutical agents that retain the analgesic properties of the known opioid, but that have reduced side effect profiles.
Additionally, dopamine, norepinephrine and serotonin are mammalian monoamine neurotransmitters that play important roles in a wide variety of physiological processes. Therefore, compounds that selectively modulate the activity of these three neurotransmitters, either individually, in pairs, or as a group, promise to serve as agents effective in the treatment of a wide range of maladies, conditions and diseases that afflict mammals due to atypical activities of these neurotransmitters. Interestingly, a significant portion of the known compounds that modulate the activity of these three neurotransmitters, either individually, in pairs, or as a group, comprise a substituted piperidine moiety.
Dopamine plays a major role in addiction. Many of the concepts that apply to dopamine apply to other neurotransmitters as well. As a chemical messenger, dopamine is similar to adrenaline. Dopamine affects brain processes that control movement, emotional response, and ability to experience pleasure and pain. Regulation of dopamine plays a crucial role in our mental and physical health. Neurons containing the neurotransmitter dopamine are clustered in the midbrain in an area called the substantia nigra. In Parkinson's disease, the dopamine-transmitting neurons in this area die. As a result, the brains of people with Parkinson's disease contain almost no dopamine. To help relieve their symptoms, these patients are given L-DOPA, a drug that can be converted in the brain to dopamine.
Norepinephrine, also called noradrenaline, is a neurotransmitter that also acts as a hormone. As a neurotransmitter, norepinephrine helps to regulate arousal, dreaming, and moods. As a hormone, it acts to increase blood pressure, constrict blood vessels and increase heart rate—responses that occur when we feel stress.
Serotonin (5-hydroxytryptamine, 5-HT) is widely distributed in animals and plants, occurring in vertebrates, fruits, nuts, and venoms. A number of congeners of serotonin are also found in nature and have been shown to possess a variety of peripheral and central nervous system activities. Serotonin may be obtained from a variety of dietary sources; however, endogenous 5-HT is synthesized in situ from tryptophan through the actions of the enzymes tryptophan hydroxylase and aromatic L-amino acid decarboxylase. Both dietary and endogenous 5-HT are rapidly metabolized and inactivated by monoamine oxidase and aldehyde dehydrogenase to the major metabolite, 5-hydroxyindoleacetic acid (5-HIAA).
Serotonin is implicated in the etiology or treatment of various disorders, particularly those of the central nervous system, including anxiety, depression, obsessive-compulsive disorder, schizophrenia, stroke, obesity, pain, hypertension, vascular disorders, migraine, and nausea. Recently, understanding of the role of 5-HT in these and other disorders has advanced rapidly due to increasing understanding of the physiological role of various serotonin receptor subtypes.
Although various methods have been reported for laboratory synthesis of piperidines, the vast majority of these methods are not suitable for a commercial-scale process. Moreover, there are no reliable stereoselective methods for the controled asymmetric synthesis of substituted piperidines. The disadvantages of the traditional synthetic methods include modest overall yields and poor stereoselectivities. Moreover, small amounts of by-products, such as undesired stereoisomers, often accumulate during the synthetic protocol, making complete purification of the final product difficult.